Level beam but floors still sagging??

[Jeff C]

I recently bought at 100 year-old Cape-style house. The floors were sagging and the culprit appeared to be the center beam, which was being supported by 3 6x6 posts that had rotted from the bottom up over the years. The worst sagging was probably about 2".

I've jacked up the beam and re-supported it with adjustable jack-posts and all seems well except that the joists don't seem to have moved with the beam. In other words, the beam is far more level and the floor directly above the beam is just as level, but the floors simply have a "speed bump" shape in them where the beam is supporting them but they quickly sag off to either side of the beam.

The joists appear to be in good condition, though they're certainly original and they are supported both by being mortised into the beam and additionally by bulky (1/4" thick?) 90-degree angle braces. I'm trying to figure out why the beam moved but the joists apparently did not.

N.B. - I did jack the beam up relatively quickly. Over the course of maybe a month. Could that have something to do with it? Help appreciated, thanks.

JPC

[tbasc]

If it were my problem, I'd look at the way the beam and joists are connected. It may also be that the joists are sagging. It may be because the joists are restrained in some way from moving. As an architect, I'm biased, but you might pay an architect or engineer to visit and consult. TB

[Joseph Meehan]

I am neither a architect or an engineer, but I agree.

[Roger Shoaf]

As others have suggested an engineer is probably worth his consulting fee, but my guess is that the joists probably have too much span and are flexing. I suspect the fix is going to be to add an additional beam midway between your existing beams and jack that up.

You might also have a problem with the condition of the joists themselves. Inspect carefully the intersection of the joists and the beam you jacked up. If the joists are cracked where they were mortised into the center beam this would explain your results.

I suspect if these are damaged, it was probably from the trip down rather than being jacked up.

You might also find a place where you can cut an inspection hole in the floor to inspect from the top down.

Be sure to post and let us know what the problem was and how you fixed it.

[Duane Bozarth]

Concur w/ others, but...I'd suspect perhaps the angle braces aren't original but were added to try to solve an earlier problem. How large are they and are they really mortised in or simply setting on a ledger (and the angle is an attempt to keep them there)? 100 yrs isn't old so could be this is dimension lumber structure? I'd get another jack and support the end near the beam of two or three and do some serious exploring of the joint technique and condition of the joists by taking one or two of the angles down (if necessary, that's the reason for the additional support, just in case it's the only thing left really holding anything) and see what's actually going on. If you raised the beam but the joists didn't move and that's anything close to 2", you've got a problem. If, otoh, they went up but not as far, it's probably not too serious. Fixing it may not be easy, however.

Have you put a line on the joists themselves, do they have pronounced bow? Did you check this before lifting so you can compare?

[Jeff C]

Sorry for the long absence, the holidays impeded my progress. Upon consultation with both a structural engineer and building contractor, it turns out the angle braces were there to fix the cracks that had developed in the joists near the tenons. When I jacked up the beam, they came up but not as far as the beam and that was due to both increased sagging as well as further cracking at the tenon joints.

The joists are hand-hewn and, interestingly, the joists spanning from the beam to the back sill are significantly larger than those spanning from the beam to the front. This problem afflicts the smaller joists much more seriously and those will need replacement while the larger, rear joists will need sistering. Thanks for everyone's thoughts on this problem.